1. Field of the Invention
The disclosed subject matter relates generally to a pump with magnetic bearings.
2. Description of the Related Art
Electrically driven pumps have been in common use for many years. One application of such an electrically driven pump is in the field of hydrocarbon service, where subsea pumps may be used. Subsea pumps have been used to pump multiphase fluids, typically including any pump-able combination of oil, gas, water and/or solids, as well as single-phase fluids, e.g. water and/or oil. Conventionally, one of three subsea pump/motor technologies are typically deployed subsea on commercial applications. Two technologies may be characterized as having a “short-fat” induction motor driving a relatively short rotor-dynamic pump (e.g., up to 14 stages), or driving a twin-screw positive displacement pump, which is also relatively short. Typically, rotor-dynamic pumps have been oriented vertically with the induction motor positioned over the pump, whereas the twin-screw pump units have been oriented horizontally. The third pump/motor technology deployed subsea employs a repackaged electric submersible pump (ESP), which may be characterized as a “long-skinny” induction motor driving a long-skinny rotor-dynamic pump (e.g., including several tens of stages). Subsea ESPs may be deployed vertically in a caisson/dummy-well or riser, or in a near-horizontal orientation proximate the seabed, (e.g., on a foundation structure or in a flowline jumper).
FIG. 1 is a representation of a prior art subsea multiphase rotor-dynamic pump/motor assembly, referred to hereinafter as a pump unit 100 that includes a pump 101 and an induction motor 116. Multiphase fluid enters the pump 101 into a flow-mixing chamber 102 via inlet 103. The mixed fluid next enters a pump compression chamber via inlet 104 where it is progressively pressurized through a series of stages comprising rotating impellers 105 and static diffusers 106. The resulting higher pressure fluid is ultimately exhausted to the downstream piping (not shown) through diffuser chambers 107 and an outlet 108.
The impellers 105 are unitized to a pump shaft 109, whereas the diffusors 106 are unitized to a pump pressure housing 110. The shaft 109 is supported by radial bearings 111, 112 and an axial bearing 113, the latter being designed to support the weight of the shaft 109 and components integrated thereto plus the thrust load developed by the pump hydraulic elements and the hydraulic piston effect associated with the barrier fluid system (BFS) acting on the shaft 109, flexible coupling 114, and an optional balance piston (not shown). Relevant design codes impose multiplication factors that add to axial bearing 113 load carrying capacity requirements. The shaft 109 is connected to an induction rotor 115 of the induction motor 116 by a flexible coupling 114 that transfers torque but not axial load. The rotor 115 is turned by the electro-magnetic forces generated by a stator 117. The rotor 115 is supported by radial bearings 118, 119 and an axial bearing 120, the latter being designed to support the weight of the rotor 115, the hydraulic piston effect associated with the BFS interaction therewith and on the flexible coupling 114, and design code multiplication factors. All the bearings are typically hydrodynamic tilting-pad mechanical bearings for which the rotating versus non-rotating elements are separated under dynamic (“hydraulic-lift”) conditions by a film from a pressurized fluid 121. Contact between bearing mechanical elements may occur whenever there is no relative movement between those elements. Fluid 121 for creating the film is provided by a BFS described in greater detail elsewhere in this document.
The barrier fluid 121 distributed widely within the pump unit 101 should ideally be maintained at a pressure greater than the outlet pressure of the pump 101 to serve its multiple functions in conventional systems, such as that illustrated in FIG. 1. The barrier fluid 121 is typically supplied from a remote location into the pump unit 100 to surround induction motor stator 117 and all of the rotating equipment except the pump hydraulics. Controlled-leakage rotating mechanical seals 122, 123 that will vent barrier fluid pressure above a certain level into the process stream are provided near both ends of the impeller stack on the shaft 109 to maintain the barrier fluid 121 in the desired areas while also creating the required higher-than-pump-outlet pressure in those areas. The pressure-bias created by the rotating mechanical seals 122, 123 is one method for excluding process fluids and associated debris and corrosion agents, etc., from sensitive areas in the pump 101 and induction motor 116. The controlled-leakage of the mechanical seals 122, 123 provides a protective fluid film and cooling effect for those seals. Because the mechanical seals 122,123 leak barrier fluid, the BFS must periodically be resupplied, resulting in undesirable monitoring and maintenance activities that directly increase operating expense. Furthermore, depending on the specific features of a supplier's motor design, the BFS may suffer an onerous requirement to be maintained dehydrated to a high-specification level.
In addition to lubricating and cooling the bearings 111, 112, 113, 118, 119, 120 and mechanical seals 122, 123, another function of the BFS is to provide electrical insulation and cooling for the stator 117 and associated items such as high-voltage power penetrators 124. The aforementioned items, especially the stator 117 generate large amounts of heat during operation. Damage resulting in system failure will occur quickly if heat beyond design capacity is not removed from the system. Owing partly to the pump and motor multiple, thick wall-section, limited externally-exposed-surface-area housings 110, 125 and 126, 127 respectively, and also to heat-transfer characteristics of the multiple materials involved, including the barrier fluid, heat transmitted naturally between the heat-generating elements and the barrier fluid 121 cannot be adequately moved by passive means alone to the environment surrounding the pump unit 100 (i.e., via conduction, convection and/or radiation). It is therefore necessary, for all but low-power systems, that barrier fluid 121 be circulated through an external long-conduit heat-exchanger, possibly including multiple flow-paths 128. Such a system typically also requires a pump to circulate the barrier fluid 121, which in FIG. 1 is satisfied by a dedicated impeller 129 unitized to the process pump shaft 109.
A typical barrier fluid system associated with prior art subsea pump systems comprises many components, some positioned proximate the subsea pump and others located on a topside (above water) facility usually several miles away. A typical BFS comprises a hydraulic power unit, fluid storage tanks, cleaning and dehydrating equipment, filters, pumps for moving fluids between various topside components and for delivering the barrier fluid to the subsea pump, flow restrictors, non-return valves, accumulators, full-bore valves, pipes and fittings, one or more lines in the subsea umbilical, pressure and temperature sensors, level-monitoring instruments, and control systems. Because several of these components are critical to the correct functioning of the system and therefore the integrity and reliability of the associated subsea pump, redundant such components are typically provided for each field application. Many of these components require periodic maintenance, and the amount and condition of the barrier fluid in the storage tank(s) must be carefully monitored and maintained at all times. Barrier fluid circulated within prior art subsea pumps and motors is also the primary means for removing heat therefrom, especially from electric motors, and there are several components associated with that function, including dedicated pumps/impellers and heat-exchanger tubes.
This section of this document is intended to introduce various aspects of art that may be related to various aspects of the disclosed subject matter described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the disclosed subject matter. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art. The disclosed subject matter is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.